1. Field of the Invention
The present invention relates generally to a piezoelectric transformer driving circuit and a driving method. More particularly, the invention relates to a driving circuit and a driving method for driving a piezoelectric transformer with sweeping a repetition frequency of a drive voltage within a predetermined sweeping range.
2. Description of the Related Art
In general, a piezoelectric transformer is an element to contact electrodes on a primary side and a secondary side to apply a voltage of resonant frequency of the transformer for resonating the transformer on the primary side and to take out a generated voltage on the secondary side generated by a mechanical vibration due to resonation. Such piezoelectric transformer is characterized by the capability of down-sizing and the reduction of thickness in comparison with an electromagnetic transformer and is drawing attention as a back-light of a display device with a liquid crystal and other devices.
As shown in FIG. 8A, the piezoelectric transformer has characteristics of a maximum transformation ratio in the vicinity of a resonant frequency f0, which is different from the characteristics of the electromagnetic transformer which has a fixed transformation ratio and a wide transmission band. For example, when the resonant frequency of the piezoelectric transformer is 117 kHz, the piezoelectric transformer has the transmission band of about several kHz. When the piezoelectric transformer is driven to obtain a given output, there is a method for making the transformation ratio variable by varying a driving frequency of the piezoelectric transformer. Conventionally, as the driving circuit of the piezoelectric element of this kind, there has been known the driving circuit disclosed in Japanese Unexamined Patent Publication No. Heisei 8-107678. The conventional circuit disclosed in the above-identified publication will be discussed with reference to FIG. 9.
As shown in FIG. 9, a driving circuit 401 is connected to a primary electrode 106 of a piezoelectric transformer 102. The driving circuit 401 amplifies a drive voltage into a voltage necessary for driving a piezoelectric transformer under control by a control signal 5050 from a frequency sweeping oscillator 405, for driving the piezoelectric transformer 102 from the primary electrode 106.
A secondary electrode 107 of the piezoelectric transformer 102 applies a voltage transformed by the piezoelectric transformer 102 to a high voltage terminals 108 of a load 103. A flow-out current from a load 103 flows into a load current comparing circuit 104 via a low voltage terminal 109 of the load 103.
In the load current comparing circuit 104, a load current is converted into a voltage and is compared with a reference voltage (VrefA) 110 corresponding to a preliminarily provided load current value. A frequency sweeping oscillator 405 determines a direction of sweeping 1040 in response to a result of comparison in the load current comparing circuit 104. Frequency sweeping operation will be discussed hereinafter with reference to the drawings.
The piezoelectric transformer 102 has frequencies of the same transformation ratio at respective one point on high frequency side and low frequency side of the resonant frequency, at which a maximum power is output. However, as shown in FIG. 8A, a power transmission ratio between input and output of the piezoelectric transformer is higher on the high frequency side than the low frequency side. Accordingly, in order to drive the piezoelectric transformer efficiently, it is preferred to operate the piezoelectric transformer on the higher side of the resonant frequency f0.
An operation of a piezoelectric transformer inverter is as illustrated in FIGS. 10A and 10B. As shown in FIG. 10A, until the input frequency of the piezoelectric transformer reaches a desired load current value, the frequency is swept from the high frequency side to the low frequency side. Then, at a timing where the desired load current value IH is exceeded as shown in FIG. 10B, a sweeping direction of the drive frequency is reversed to increase the frequency. Then, at a timing where the load current is varied across the desired load current value again, the sweeping direction of the drive frequency is reversed to decrease the frequency. By repeating this operation, the load current is held in the vicinity of the desired value. The foregoing operation is performed at a frequency higher than the resonant frequency.
On the other hand, when the desired load current is not reached, the frequency sweeping direction is reversed to increase the drive frequency at the lower limit of a preliminarily set frequency sweeping range by a frequency sweeping oscillator 405, as shown in FIG. 11A. At a timing where an upper limit of the preliminarily set frequency sweeping range is reached, sweeping of the frequency is initiated again to decreased until the desired load current IH is reached, as shown in FIG. 11C.
In this case, for example, as shown in FIG. 11B, when a control voltage value of the control voltage of the oscillator 522 reaches an upper limit voltage VH, a reset operation for an integrator 519 may be performed. Thus, the control voltage value repeats sweeping between the lower limit voltage VL and the upper limit voltage VH. The upper limit voltage VH may be set at 2V, for example, and the lower limit voltage VL may be set at 0.5V, for example. These values may be adjusted depending upon characteristics of the piezoelectric transformer and/or use condition thereof.
By setting of speed of elevating the drive frequency in the foregoing operation, operation for stabilizing the load current in the vicinity of the desired value can be performed at higher speed. In conjunction therewith, occurrence of resonation at the abnormal level can be prevented.
On the other hand, as shown in FIG. 8A, an upper limit fU of the frequency sweeping range is set to be higher than the resonant frequency f0, and a lower limit fD of the frequency sweeping range is set to be lower than the resonant frequency f0. Then, sweeping operation is performed between the upper limit fU and the lower limit fD of the frequency sweeping range as shown by arrows Ya1 and Ya2.
Here, the frequency sweeping oscillator 405 may be constructed with the integrator 519, a comparator 520 and an oscillator 522, as shown in FIG. 12. It should be noted that the integrator 519 is constructed to elevate an output voltage thereof at a given rate depending upon an output of the load current comparing circuit 104. The output voltage of the integrator 519 is input to the oscillator 522. The oscillator 522 outputs a frequency pulse inversely proportional to an input voltage value. The oscillator 522 is a voltage controlled oscillator which outputs the frequency pulse to the drive circuit 401. The oscillator 522 is constructed with a charge-and-discharge circuit 521, a resistor 524 and a capacitor 523. The oscillator 522 determines a current value to be charged and discharged to and from the capacitor 523 by an input voltage value from the integrator 519 and a resistance value of the resistor 524 so that the charge-and-discharge frequency of the capacitor 523 becomes an oscillation frequency.
The comparator 520 receives the output of the integrator 519 to compare with a reference voltage Vmin. Then, when the output voltage of the integrator 519 is higher than the reference voltage Vmin, a reset signal 5200 is output to the integrator 519. An output voltage of the integrator 519, to which the reset signal 5200 is input, becomes a minimum potential. By this, the drive frequency of the piezoelectric transformer 102 becomes the upper value of the sweeping range.
The oscillator 522 and the integrator 519 performs integrating operation and oscillating operation with charging and discharging the capacitor not shown. Namely, the integrator 519 performs integrating operation by charging and discharging the capacitor not shown. However, upon responding to input of the reset signal 5200, the integrator 519 drives an internal switch to make a not shown capacitor to discharge. By this, the operation shown in FIGS. 11A to 11C is performed.
On the other hand, the piezoelectric transformer has a characteristics to vary the transformation ratio by the load impedance. For example, referring to FIG. 13 showing a relationship between the transformation ratio and a drive frequency, at no load condition, the piezoelectric transformer has quite high transformation ratio to output quite high voltage. Namely, as shown in FIG. 13, a greater impedance value of the load causes a higher transformation ratio.
At this time, as shown in FIG. 14, a proportional relationship is established between the output voltage and a vibration velocity. Therefore, since the piezoelectric transformer is broken, a protection circuit becomes necessary. An output voltage comparator circuit 206 in FIG. 9 has a function for voltage division and rectification of the voltage output to a secondary electrode 107 of the piezoelectric transformer. Then, the divided and rectified voltage is compared with a reference voltage (VrefB) 212 to apply a result of comparison 2060 to the frequency sweeping oscillator 405.
As shown in FIG. 8B, when the frequency sweeping oscillator 405 has a function to switch the sweeping direction of the frequency from downward direction (direction of arrow Yb1) to upward direction (direction of arrow Y2) when the result of comparison of the output voltage comparator circuit 206 shows that the output voltage exceeds a preliminarily set reference voltage VrefB. By this function, when judgement is made that the load becomes open in certain reason to exceed the preliminarily set output voltage, the drive frequency of the piezoelectric transformer transit to a condition of low transformation ratio to lower the output voltage. By this function, by abrupt elevation of the output voltage of the piezoelectric transformer associating with abrupt increase of the load impedance, the piezoelectric transformer may prevent possible occurrence of breakage due to excessive vibration. A time division drive control circuit 306 in FIG. 9 generates a driving stop signal 4010 controlling an output duty according to a duty control voltage (Vduty) 307 at a low frequency as low as 1/100 with respect to the drive frequency of the piezoelectric transformer. The drive circuit 401 has a function to stop the drive voltage to be applied to the primary electrode of the piezoelectric transformer in response to the driving stop signal 4010 from the time division drive control circuit 306 to control an average output power of the piezoelectric transformer 102.
On the other hand, the frequency sweeping oscillator 405 stops sweeping of frequency in response to the driving stop signal from the time division drive control circuit 306 and shifts the frequency to a frequency slightly higher than the piezoelectric transformer drive frequency at the time of stopping. Upon initiation of frequency sweeping again in responsive to termination of the driving stop signal 4010 from the time division drive control circuit 306, the frequency sweeping oscillator 405 performs sweeping in a direction to lower the frequency from the frequency slightly higher than the drive frequency at the timing of stop sweeping. By this function, upon resumption of sweeping in the time division driving, a possible problem occurring in the load current control operation due to delay of the output voltage in relation to the driving waveform due to mechanical resonance, can be prevented.
The piezoelectric transformer driven circuit constructed as set forth above can be driven at high efficiency. In conjunction therewith, prevention of breakage due to opening of load which is a particular problem of the piezoelectric transformer, and prevention of a problem to be caused by interference of the average output power control for the display can be achieved. Accordingly, problems which hinders practical use of the piezoelectric transformer can be solved and inexpensive drive circuit can be realized.
However, in the foregoing prior art, there are two problems.
First problem is the possibility of occurrence of individual circuit, in which an effective value of vibration velocity at no load operation becomes high. Such circuit continues operation in a condition close to extreme of performance of the piezoelectric transformer per se, generated heat becomes large to potentially cause degradation of characteristics. When the vibration velocity of the piezoelectric transformer becomes high, abrupt heating can be caused as one example shown in FIG. 15 to reach Curie point to weaken polarization.
When load becomes open in certain reason and the output voltage comparing circuit 206 makes judgment that the output voltage exceeded the preliminarily set output voltage Vr, the frequency sweeping direction is switched from the downward direction to the upward direction. Then, the drive frequency is reversed to lower at a timing where the upper limit fU set in the frequency sweeping oscillator 405 is reached. This sequence of operation is shown in FIG. 16.
Referring to FIG. 16, in the sweeping operation of the drive frequency in upward and downward direction, the upper limit fU of the frequency sweeping range is determined by setting of a constant in the frequency sweeping oscillator 405. The oscillator 522 in the frequency sweeping oscillator 405 is frequently the voltage controlled oscillator circuit (VCO), oscillation frequency of which is determined depending upon the applied voltage value. The voltage controlled oscillator circuit is generally small in scale of the circuit and simple in control method.
FIG. 17A shows one example of a relationship between an oscillator control voltage V and the driving frequency, and a further relationship with the output voltage of the piezoelectric transformer at respective timings.
On the other hand, a capacity value of a charge-and-discharge capacitor of the voltage controlled oscillator circuit and a resistance value for determining a constant current value and so forth have tolerances. This deviation can cause problem in the inverter characteristics. This will be discussed hereinafter.
For example, it is assumed that the characteristics shown in FIG. 17A represents a data of a center value in designing in no load operation. A resonant frequency, at which the transformation ratio of the piezoelectric transformer mounted on the inverter becomes maximum, becomes a frequency lower than 119 kHz. In case of FIG. 17A, it is so designed that the upper limit of the frequency sweeping range becomes 125 kHz when the oscillator control voltage is 0.5V. By elevating the oscillation control voltage from this condition, the oscillation frequency is lowered and the output voltage is increased.
When the output voltage of the piezoelectric transformer exceeds 18 kVo-p, the output voltage comparing circuit 206, is constructed to apply the signal for reversing the frequency sweeping direction to the frequency sweeping oscillator. Then, when the drive frequency is lowered down to 119 kHz, at which the output voltage becomes 1.8 kVo-p, the sweeping direction is reversed and the oscillator control voltage is lowered down to 0.5V. At the timing where the oscillator control voltage becomes 0.5V, the foregoing sequence of operation is repeated again.
Here, one, in which the frequency sweeping operation shown in FIG. 17A is re-written on transformation ratio-drive frequency curve, is shown in FIG. 18A. In FIG. 18A, the resonant frequency is f0. Then, between f1 at 119 kHz and f2 at 125 kHz, sweeping operation of the drive frequency is performed as shown by arrow Ya.
However, due to fluctuation of characteristics of respective parts forming the oscillator, individual circuit causing behavior shown in FIG. 17B can occur even when parts having the same nominal values are used. The individual circuit has the oscillation frequency of 120 kHz when the oscillator control voltage is 0.5V.
Since the piezoelectric transformer has strong resonant characteristics, the output voltage as driven at 120 kHz becomes significantly greater than the former driven at 125 kHz as design center value. From this condition, by elevating the oscillator control voltage, the oscillation frequency is lowered and the output voltage is increased. Similarly to the former, at a timing where the drive frequency reaches 119 kHz, the output voltage of the piezoelectric transformer becomes 1.8 kVo-p. Then, the output voltage comparing circuit 206 feeds a sweeping direction reversing signal to the frequency sweeping oscillator 405. By this, the oscillator control voltage returns to 0.5V, the drive frequency returns to 120 kHz. At this timing, again the oscillator control voltage is elevated and the drive frequency is lowered.
As shown in FIG. 17B, in such condition, since the sweeping range of the drive frequency is narrow and operation is continued in a range where the transformation ratio of the piezoelectric transformer 102 is high, high voltage output is continued. The characteristics re-written the frequency sweeping operation shown in FIG. 17B on the transformation ratio-drive frequency curve, is shown in FIG. 18B. In FIG. 18B, the resonant frequency is f0. Between f1 at 119 kHz and f3 at 120 kHz, sweeping operation of the drive frequency is performed as shown by arrow Yb.
On the other hand, as shown in FIG. 14, there are substantially proportional relationships between the vibration velocity of the piezoelectric transformer and the output voltage. Therefore, vibration velocity on the horizontal axis in FIG. 15 can be replaced with the output voltage of the piezoelectric transformer. On the other hand, the output voltage value can be converted into corresponding drive frequency. In FIG. 18C, a relationship between the drive frequency and the elevation of the surface temperature of the piezoelectric transformer in the procedure set forth above, is shown.
In FIG. 18C, a temperature corresponding to the drive frequency f1 (119 kHz) is represented by Tf1, a temperature corresponding to the drive frequency f2 (120 kHz) is represented by Tf3, and a temperature corresponding to f3 (125 kHz) is represented by Tf2. In case of FIG. 18A, the sweeping operation is performed in the direction shown by the arrow Ya, and in case of FIG. 18B, the sweeping operation is performed in the direction shown by the arrow Yb.
Referring to FIG. 18C, it may be appreciated that the drive frequency having low and narrow frequency sweeping range as shown in FIG. 17B, is driven in a range where elevation of the surface temperature of the piezoelectric transformer is high in comparison with the drive frequency having frequency sweeping range of the design center as shown in FIG. 17A.
Essentially, the reason of designing to perform operation shown in FIG. 17A is as follow. Namely, when the circumference is low temperature or dark, an impedance of a cold-cathode tube to be a load becomes higher than a normal impedance. The piezoelectric transformer is required to output a voltage requiring for initiation of lighting of the load in this condition. However, if the output value is maintained constantly, vibration velocity of the piezoelectric transformer per se becomes high to continue operation in the condition close to an extreme of performance thereof. Therefore, degradation of characteristics is caused due to elevation of temperature in the piezoelectric transformer.
Therefore, sweeping is initiated from a high frequency having low vibration velocity. By returning the frequency higher at a timing where sweeping of frequency is lowered to obtain the output voltage necessary for lighting the load, lowering of effective value of the vibration velocity than necessary voltage continuous with maintaining outputting of the necessary voltage, is realized.
However, in the operation state shown in FIG. 17B, the effective value of the vibration velocity becomes high. Therefore, since the piezoelectric transformer per se continues operation in a condition close to the extreme of performance, temperature elevation becomes large to cause degradation of the characteristics.
The second problem is that it is difficult in the light of design to shift the upper limit value of the sweeping frequency range of the frequency sweeping oscillator 405 upon resetting of the integrator 519 in the frequency sweeping oscillator 405. Namely, by shifting the setting of the upper limit value of the sweeping frequency range upon resetting of the integrator 519, higher in design, the foregoing first problem may be resolved. However, the following problem may be encountered.
A driving waveform fed from the driving circuit 401 to the piezoelectric transformer 102 has a characteristics for transmitting a power only in narrow band of the piezoelectric transformer. Therefore, since high harmonic component is not used, efficiency becomes high as driven by a sine wave. Therefore, wave shaping is performed to obtain the sine wave at the desired load current output frequency fH in FIG. 8A. In the light of simplification of a circuit construction, a method for shaping into the sine wave by an equivalent circuit of the piezoelectric transformer and a coil on a drive circuit side. Therefore, at higher frequency beyond the desired load current output frequency, error of timing of zero volt switching becomes greater to destroy the sine waveform. Then, degradation of conversion efficiency in the drive circuit 401 is caused to increase heating of the circuit parts to cause a problem in reliability.
On the other hand, the piezoelectric transformer 102 determines its own resonant frequency by the shape thereof. At the same time, frequencies to cause vibrations in the lateral direction or torsional mode other than normal orientation are also determined. If the piezoelectric transformer 102 is in operation at the frequency where lateral vibration and torsional vibration are caused, conversion efficiency in the piezoelectric transformer 102 is degraded to generate heat to cause reliability.
Referring to FIG. 19, the frequencies identified by "x" are the frequencies causing lateral vibration or torsional vibration. As can be clear from FIG. 19, at a wider effective sweeping frequency range, greater number of frequencies to cause lateral vibration or torsional vibration may be included.
A solution for the first problem is to shift the upper limit value of the frequency sweeping range higher. To the contrary to this, a solution for the second problem is to shift the upper limit value of the frequency sweeping range lower. Therefore, trade-off is caused in the solutions of the first problem and the second problem to make designing difficult.
It should be noted that as the prior art relating to the frequency sweeping method of the inverter, the following are known. At first, in Japanese Unexamined Patent Publication No. Heisei 8-251929, an object is to constantly perform efficient operation even when the input power source or load is fluctuated. For this purpose, the output of the piezoelectric transformer reaching the predetermined value is detected, and subsequently, with sweeping the oscillation frequency of the drive frequency control circuit by the frequency sweeping circuit across the resonant frequency of the piezoelectric transformer, efficiency is detected by an efficiency calculation circuit. Then, a frequency where the efficiency becomes maximum is detected for subsequently driving the frequency where the efficiency becomes optimal.
On the other hand, Japanese Unexamined Patent Publication No. Heisei 6-96887 discloses a system including a direct current power source, in which a commercial power source is rectified and smoothed and is directed for prevention of abnormal oscillation even when the voltage becomes lower than or equal to a predetermined value. The disclosed system monitors an output voltage of the direct current power source for switching a resistance value of the oscillation circuit so that the oscillation frequency will not be lowered less than or equal to the predetermined frequency.
Also, Japanese Unexamined Patent Publication No. Heisei 5-219730 is directed to control an output current constant with respect to an environmental temperature. A power source voltage and a load is provided with an oscillator variable of oscillation frequency on the basis of the output of the current detection means.
However, none of these publications are directed to protecting the operation of the piezoelectric transformer, and has means for performing protection, and thus the foregoing problems cannot be solved.